# Alkaline battery noise varies with temp

I had an 9V alkaline battery connected to a resistive bridge, that stepped out the voltage to several analog channels. I was testing the analog channels over temperature and when it got below 10C I noticed that the battery voltage noise went from >1uV to 10's mV. As I have always thought that batteries were a stable source I started checking my analog electronics, only to find it was the battery.

Has anybody characterized this noise or the temperature it starts at?
Where does it come from (what physical processes)?
Does this apply to all battery chemistries (Do all battery types get noisy with lower temps)?

Edit - More stuff:
This is not mechanical, a test engineer and I ruled that out. The electronics are not at the same temperature, and are not powered by the battery. The battery is a reference. There are sensors that we use that we normally take down to temperature that the analog electronics are connected to and there is no problem with noise with the regular sensor. The noise is coming from the battery

Edit - Final word: So you don't have to read through loads of comments, I'll post the result here. When I woke up this morning I thought I would heed the council of a few users and double check the mechanical setup. I suggested that the tech look things over and redo the solder joints with leaded solder instead of the lead free. After that things worked great, I had less than 1uV noise going down in temp. So I apologize for not listening to comments about mechanical.

• I'd guess that you have a connection which is mechanically marginal, and the lower temperature causes it to loosen up and become somewhat intermittent. Fortunately, 9V batteries are cheap. Replace it and check your connections. – WhatRoughBeast Mar 8 '16 at 0:01
• Interesting observation +1 . It has been said that batteries dont make noise .What is the spectra of your alleged noise ? I geuss that you will check and recheck your test setup .If the noise persists then you have discovered something. – Autistic Mar 8 '16 at 0:26
• It could be that the effective impedance of your battery increases as temperature drops (most don't "like" cold) and that may mean an increased noise. But first rule out some other aspect of your experiment failing! – Chris Stratton Mar 8 '16 at 1:33
• Interesting question. I think it's generally assumed batteries aren't noise sources, not known or proven. As chemical reaction rates reduce with cold, perhaps it isn't surprising that the statistics change too - if the reaction is exothermic, then local heating may locally increase rate -> LF noise. If so, I would expect it to be very dependent on battery chemistry as well as temperature : you may have stumbled on an interesting research area, and I'd be interested in the results! – Brian Drummond Mar 8 '16 at 12:13
• Forgive me for being so out of it here, but, I honestly believed as I was taught that any source of resistance at any temperature could and should experience thermal noise. Are batteries different because of their chemistry, offsetting the noise? Or a larger thermal mass that is resistant to those kinds of fluctuations? Or...? Just curious. – Sean Boddy Mar 8 '16 at 15:42

Considering that noise variation with temperature is a fundamental property of matter, all the things (which includes alkaline batteries) will have noise proportional to temperature. All resistances have thermal noise, and all batteries have resistance, and their noise is more or less from that internal resistance. The voltage noise of a battery (or resistor) is: $$V_{noise}=\sqrt{\frac{4hfR\Delta v}{e^{\frac{hf}{kT}} -1}}$$

where h is planck's constant, f is the frequency, R is the internal resistance of the cells or cell, ∆v is the bandwidth, k is Boltzmann's constant, and T is temperature in kelvin. As you can see, lowering the temperature lowers the noise. This is true for everything, there is nothing happening here unique to batteries. This noise is called Johnson-Nyquist noise.

As for which chemistry has the lowest noise, there is no meaningful difference in theory. In practice, Nickel-Cadmium cells have the lowest voltage noise. However, this is purely due to that chemistry also having the lowest internal resistance. As you can see from the earlier equation, lowering resistance will lower noise over all. Alkaline cells have relatively high internal resistance, so it is not surprising they would be noisier as a chemistry. Note that this means cell size is as important to the voltage noise as cell chemistry. Larger cells have lower internal resistance and therefore lower noise.

But don't take my word for it. Take NIST's. They did a study on the noise of batteries, and there are nice graphs for those curious in that paper, but after substantial measurements all the way down to the thermodynamically limited noise-floor, they concluded that battery voltage noise is essentially in agreement with the expected Johnson-Nyquist thermal noise one would expect from the cell's internal resistance.

Edit: Whoops, I forgot that the entire question was about the noise increasing once it got cold enough. A battery's internal resistance increases as it gets cold, and decreases as it gets warm. This mechanism is chemical in nature, and likely could vary between different constructions of the same chemistry. In general, temperature can increase the internal resistance a lot once you get cold enough. The internal resistance is ultimately determined by the rate the chemical reaction can occur, and the colder the battery, the slower the reaction. It's a safe bet to look at a cell or chemistry's internal resistance vs. temperature, this should give you a good idea of how warm you need to keep the cell. There is going to be a 'sweet spot' where the noise is lowest. Warmer and the temperature increases the noise more than the internal resistance decreases, colder and the internal resistance increases more than the noise decreases.

EDIT2: It looks like an alkaline cell's internal resistance doubles (or at least an AA cell) going from 20 degrees C to 10. This is far too small to account for the several orders of magnitude noise increase.

Sorry. Something weird is going on. Thermocouple effects perhaps?

• I think the OP is observing a noise increase as the temperature went down, rather than up, and specifically a marked jump as the 10°C threshold was passed. – scanny Mar 8 '16 at 20:37
• You're absolutely right scanny. I've ammended my answer, though I don't think it really counts as an answer anymore. It's too long for a comment - it might be useful but shouldn't get any votes. I am not sure what is causing such odd behavior. – metacollin Mar 8 '16 at 20:55
• I think this is worthwhile answer as it helps frame the problem and provides a proper background for other comments which are wrong i.e. "batteries don't have noise" etc. It may not provide the answer but it moves things forward. – placeholder Mar 8 '16 at 21:11
• So, I redid the mechanical connection with leaded solder, and the noise at low temperatures went away, which means batteries still function like I would expect (and you would expect). Since you put the most thought into your answer (and I figure that this is how batteries should preform), you get the prize. – Voltage Spike Mar 8 '16 at 22:03

Batteries do have noise, it's just the thermal noise of the ESR, which is almost always less than other noise sources. What is being missed here is that in a very short temperature span the noise level jumps 3-4 orders of magnitude. See @metacollins answer for more details.

Even given electrochemical equations, this is far far larger an effect than should be expected, see Arrhenius equations etc. For this to happen means that the activation energy of the system is close to the 0.026 eV at room temperature.

My spidey sense tells me that this might be a physical change in the battery due to construction effects. If the battery is made with a granular structure, as the cell contracts one can have very different conduction paths through the cell, with an abrupt transition in the resistance of the cell due to stress/strain within the cell.

If this hypothesis is right, one would expect the increased noise level to have flicker like components (i.e. 1/f noise behaviour) in it's frequency spectrum. Long conduction paths ways that cross grain boundaries typically have this sort of signature.

Additionally you should be able to measure the cells change of resistance with temperature.

Of course if this is a production design you're now going to have verify that this is reproducible and thus specify this as a parameter in your BOM.

If my guess is right then this might actually just be one bad cell.

• I did use different batteries and some right out of the box with the same result. I'm going to look at the mechanical one last time to rule that out and make everybody (including myself) happy. – Voltage Spike Mar 8 '16 at 21:36

The noise is more likely coming from your circuit's VCC line not really the battery itself. With an increasing battery impedance the VCC noise will become more prevalent as it would no longer have the original low impendence path to ground, (through the battery). This is like placing a higher and higher resistor inline with your battery. To reduce the noise you could place a moderate value ceramic (1uf or so) directly across the battery or at the PCB battery connection points. This would lower the effective battery impedance seen by VCC and should reduce the higher frequency noise. If the noise is a lower frequency then an electrolytic cap can also be added in parallel with the ceramic cap. A cold battery or a partially depleted battery can show increased series impedance.

• The noise is not coming from the VCC line (I have really, really nice regulators to ensure this doesn't happen). Plus if it was coming from my VCC line, the noise would not be responsive to temperature. Resistors have less noise with temperature, and resistors don't have mV of noise (unless you would be talking about resistances greater than air). The seebeek effect (thermocoupules) gets lessened with temperature. I know I can lessen the noise with caps. I also know that the series resistance increases in a battery. – Voltage Spike Mar 8 '16 at 5:06
• My electronics are not using the VCC line for power – Voltage Spike Mar 8 '16 at 5:07
• What I am interested in is knowing exactly why this happens, and when it happens with temperature (I want to know the temp I have to worry about it). Batteries are a great low noise source, but apparently not with temperature, what if there is a better battery chemistry that I could use to test with at a lower temperature? – Voltage Spike Mar 8 '16 at 5:08
• Don't you have the 9v points of the bridge connected to some type of ADC high or low vref points? They may not be labeled as a VCC but they can still transfer noise if not properly bypassed. There is no reason for a simple alkaline battery to create active noise (especially in the mv range), apart from working near hi-power radio stations, thermal/mechanical creep, or some type of thermal couple effect on the contacts. Try adding small value resistors to both the + and - terminals to find what side the noise is being generated from. Detect noise levels using a differential probe. – Nedd Mar 8 '16 at 7:20
• If you don't understand the experiment, let me explain again. I am measuring the battery with an analog circuit with gain connected to ADC's. There is no noise until I take the battery and bridge down in temperature, when I bring the temperature back up the noise goes away. This is an A to B, B to A testing situation where my unknown parameter (the noise) is dependent directly on temperature. If I was getting noise from an unknown source, it would not vary with temperature. Furthermore, I can remove the battery from the unit under test and take it down in temperature and see no temp noise. – Voltage Spike Mar 8 '16 at 7:31